Remove the hard gem dependency on Curb

Curb sucks. There's no reason to introduce this C-extension dependency to a project unless the end-user really wants it. And if you really want it, it's easy. add `gem 'curb-fu` just before `fastly` in your Gemfile. 
What do you think of trying to load it?
```rb
begin
  require 'curb-fu'
rescue LoadError
  # No curf-fu. Oh well
end
```
That way, people can just add it to their `Gemfile` and don't have to also add `require` line.